The neuropeptides Orexin-A (OxA) and Orexin-B (OxB) (also known as Hypocretin-1 and Hypocretin-2) originate from the same prepro-peptide, which is expressed exclusively in the hypothalamus (1). Cleavage of the prepro-peptide (prepro-orexin) yields OxA a 33 amino acid polypeptide which is extensively post-translationally modified (C-terminal amidation, N-terminally cyclised with a pyroglutamyl residue). OxA shares a 46% sequence identity with OxB which is a 28 amino acid, C-terminally amidated linear polypeptide which likely forms a helical secondary structure (3).
The fully functional mature peptide neurotransmitters act as agonists on the orexin-1 (OX1) and orexin-2 (OX2) 7-transmembrane G-protein coupled receptors (also known as HCRTR1 and HCRTR2) that, like the orexin neuropeptides, share a high sequence homology across species (2, 6). OX1 binds both OxA and OxB, albeit, with differential affinity (OxA has >10-fold higher affinity than OxB). On the contrary OX2, which shares a 64% sequence identity with OX1, binds both polypeptides with nearly equivalent affinity (2). The primary G-protein mediated mechanism through which both receptors act is Gq/11 activation of phospholipase C catalysing the liberation of inositol-1,4,5-triphosphate (IP3), which in turn acts on IP3 receptors to release calcium from intracellular stores. OX2 has also been reported to modulate cAMP levels via activation of Gs and Gi and OX1 appears capable of signalling through Gi/o to also modulate cAMP levels (5, 8). The high degree of sequence similarity in the peptides and receptors across species translates into similar in vitro pharmacology (7).
The hypothalamus, where orexin is predominately expressed, regulates a broad array of physiological and behavioural activities. Orexin expression in this brain structure has been mapped immunohistochemically to only a very restricted number of neurons that reside specifically in the perifornical (50%), lateral and dorsomedial areas (4). The projection fields of these neurons have been identified in numerous brain regions, including the cortex, thalamus, hypothalamus, brainstem, and spinal cord, but not the cerebellum (9). This extensive coverage of the brain suggests that the orexin ligand/receptor system is implicated directly or indirectly in the regulation of multiple brain functions. Notably, knockout experiments in mice suggested that the orexin system is a key regulator of behavioural arousal, sleep and wakefulness. Indeed, the observed phenotype in orexin knockout mice was very similar to that of narcolepsy in humans (10, 11). Narcolepsy in humans is a chronic and disabling disorder characterized by excessive sleepiness during the day, fragmented sleep and cataplexy. Studies in dogs have linked the cause of the disorder to the disruption of the OX2 gene or a loss of orexin peptide expression (12). Further supporting evidence that in particular the disruption of OX2 function and or the absence of mature OxB ligand are associated with narcolepsy came from studies in knock-out mice (17). Subsequent clinical studies comparing the levels of OxA in the cerebrospinal fluid of narcoleptic patients to normal individuals confirmed that the disruption of the orexin system shows a causal relationship with the occurrence of narcolepsy in humans (13). Additional studies in unusual early onset human narcolepsy resulted in the identification of a mutation in the orexin gene that further strengthened the link between narcolepsy and the orexin system in humans (14). More recently, clinical data demonstrating the pharmacological relevance of the orexins in CNS disorders has emerged. Most notably, clinical trials with small molecule dual OX1 and OX2 antagonists (DORAs) such as BELSOMRA® (Suvorexant), have clearly demonstrated the potential utility of such agents in treating sleep disorders (15, 16, 18). These data together with the pre-clinical evidence presented above clearly implicate OX2 in sleep regulation.
The differential brain expression of OX1 and OX2 coupled with the diversity of neuro-biological effects attributed to the orexins strongly suggests drugs modulating OX1 or OX2 will elicit different biological effects. To this end, recent reports linking the OX1/OxA system specifically to feeding and behavioural disorders are important.
Given that prepro-orexin mRNA levels are mainly found in the lateral and posterior hypothalamus, areas of the brain classically implicated in the regulation of food intake and energy balance/body weight, a link between the orexin system and feeding behaviour is not unexpected (19). The role of the OX1/OxA system in such functions has been strengthened by a series of pre-clinical studies. Thus intracerebroventricular (i.c.v.) administration of OxA (20) has been shown to induce feeding and specific anti-orexin antibodies dose-dependently suppress food intake (21). In particular, the latter study indicates that orexin receptor antagonists should have a beneficial effect on orexin stimulated feeding. This hypothesis is supported by independent in vivo studies, which clearly identify OX1 as the dominant receptor of the orexin system in the regulation of food intake and energy balance. Thus, experiments conducted with selective OX1 and OX2 receptor antagonists have shown that OX1 selective compounds alter food intake and energy balance in circumstances of concurrent exposure to stress (22, 23). The dominant effect of the OX1 on regulating feeding behaviour and energy balance is further supported by observations which show that OX1 expression is selectively up-regulated in response to fasting, whereas those of OX2 are unaffected (24). Finally, studies with an OX1 specific antibody strongly suggests that a selective OX1 antagonist should suppress food intake and thus have potential therapeutic utility for the treatment of feeding related disorders such as binge eating or obesity.
Elevated OX1 levels have also been associated with psychiatric conditions including schizophrenia, anxiety and mood disorders, panic attacks, reward seeking behaviours and addiction (25, 26, 27). Studies with selective OX1 antagonists (SB334867, SB408124) clearly demonstrated a beneficial effect in a clinically relevant animal model of panic thus implying that OX1 antagonist could provide a novel therapeutic approach for the treatment of panic disorders (27).
Indirect evidence for the involvement of the orexin system in reward seeking behaviour comes from studies which show that orexinergic neurons project to reward associated brain regions such as the nucleus accumbens and ventral tegmental area (28). Direct experimental evidence comes from studies involving the intracerebroventricular (icy) infusion of orexin, which led to a dose-dependent reinstatement of cocaine seeking. The work by Boutrel et al. also links stress pathways to the effect of orexin on addiction and reward (29). Notably, stress is considered a prominent stimulus for relapse in abstinent addicts (31). The link between stress, addiction and orexin was further strengthened by pharmacological studies in a foot-shock model. These showed activation of orexin neurons in specific areas of the posterior and dorsomedial hypothalamus, which are particularly associated with stress but not the lateral hypothalamus, which has a strong link to reward (32). Moreover orexin as a mediator of stress-induced reinstatement of addictive behaviour was also shown for alcohol seeking (30). Importantly the effects of stress induced reinstatement of alcohol and cocaine seeking in animal models can be attenuated with the selective OX1 antagonist SB334867 supporting the therapeutic use of OX1 selective antagonists in these conditions (29, 30).
Finally the Orexin/OX1 pathway has been implicated in nicotine self-administration (33, 34) and re-instatement of nicotine seeking (35, 36). Such data suggest that OX1 antagonists could find utility as smoking cessation therapies.
Taken together the orexin system, in particular the OX1 pathway may be considered a target for the treatment of reward seeking behaviours, addiction and related disorders.
There is therefore a need for compounds capable of attenuating orexin-1 (OX1) activity. There is a further need for compounds capable of selectively modulating orexin-1 (OX1) function.